1. Field of the Invention
The present invention relates to an apparatus and method for estimating a frequency offset in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Because Orthogonal Frequency Division Multiplexing (OFDM) supports transmission throughput and use efficiency of a high frequency band, it is one of multiplexing systems currently being widely used. The OFDM system is very sensitive to synchronization errors. Specifically, the OFDM system may not maintain orthogonality between subcarriers when a frequency offset is present and therefore system performance is severely degraded. For this reason, many methods have been proposed to estimate the frequency offset.
The following methods observe phase variations in a symbol and estimate a frequency offset. The first method computes a correlation using characteristics of a pattern in which a Cyclic Prefix (CP) is equal to a particular part of a transmitted symbol and estimates a frequency offset in the time axis. The second method computes a phase difference by comparing phases of two half parts of one OFDM symbol in which the first half part is repeatedly transmitted in the second half part, and estimates a frequency offset. The third method transforms two repeated OFDM signals to frequency domain signals, observes a phase difference between the two frequency domain OFDM signals, and estimates a frequency offset. The fourth method transforms, to frequency domain signals, two signals obtained by sampling one OFDM symbol at a double clock frequency of a system bandwidth, computes a phase difference by comparing phases of the two frequency domain signals, and estimates a frequency offset.
Next, the above-described frequency offset estimation methods will be described in more detail.
FIG. 1 is a schematic block diagram illustrating a conventional frequency offset estimator in an OFDM system that estimates a frequency offset using the CP in the time axis.
Referring to FIG. 1, a Radio Frequency (RF) receiver 101 converts an RF signal received through an antenna (ANT) to a baseband frequency signal according to a frequency down-conversion process. A CP phase comparator 102 computes a phase difference by comparing phases of a CP and transmitted data with the same pattern as the CP in the received signal output from the RF receiver 101. An accumulator 103 accumulates and averages outputs of the phase comparator 102. A phase detector 104 detects a phase from an output signal of the accumulator 103, and computes and outputs a frequency offset. After the frequency offset output from the phase detector 104 is synchronized with a system clock, it is used to control the RF receiver 101 and so on.
The above-described method using a CP correlation may be efficient in the case where a system channel is ideal, that is, in the case of Additive White Gaussian Noise (AWGN). However, this method severely degrades performance when a signal-to-noise ratio increase due to Inter Symbol Interference (ISI) of a CP caused by a delay of a front symbol in a frequency selective fading channel in which the system actually operates.
FIG. 2 is a schematic block diagram illustrating another conventional frequency offset estimator in the OFDM system that transmits one OFDM symbol in two repeated half symbols, computes a phase difference by comparing phases thereof, and estimates a frequency offset.
Referring to FIG. 2, an RF receiver 201 converts an RF signal received through an antenna (ANT) to a baseband frequency signal according to a frequency down-conversion process. A CP remover 202 removes a CP from the received signal output by the RF receiver 201, extracts only transmitted data, and outputs the extracted data to a repeated-symbol phase comparator 203. The repeated-symbol phase comparator 203 computes a phase difference by comparing half symbols repeated in one OFDM symbol and outputs the computed phase difference to an accumulator 204. The accumulator 204 accumulates and averages outputs of the repeated-symbol phase comparator 203. A phase detector 205 detects a phase from an output signal of the accumulator 204, and computes and outputs a frequency offset. After the frequency offset output from the phase detector 205 is synchronized with a system clock, it is used to control the RF receiver 201 and so on.
There is advantageous that the method using two repeated half symbols as described above has less ISI than the CP correlation method. However, system throughput can be degraded because the same data is repeatedly transmitted.
FIG. 3 is a schematic block diagram illustrating another conventional frequency offset estimator in the OFDM system that observes frequency domain signals of two repeatedly transmitted OFDM symbols, computes a phase difference by comparing phases thereof, and estimates a frequency offset.
Referring to FIG. 3, an RF receiver 301 converts an RF signal received through an antenna (ANT) to a baseband frequency signal according to a frequency down-conversion process. A CP remover 302 removes a CP from the received signal output from the RF receiver 301, extracts only transmitted data, and outputs two OFDM symbols to a Fourier transformer 303. The Fourier transformer 303 transforms time domain OFDM signals to frequency domain OFDM signals, and outputs the frequency domain OFDM symbols to a repeated-symbol phase comparator 304. The repeated-symbol phase comparator 304 computes a phase difference by comparing phases of the two repeated frequency domain OFDM symbols, and outputs the computed phase difference to an accumulator 305. The accumulator 305 accumulates and averages outputs of the repeated-symbol phase comparator 304. A phase detector 306 detects a phase from an output signal of the accumulator 305, and computes and outputs a frequency offset. After the frequency offset output from the phase detector 306 is synchronized with a system clock, it is used to control the RF receiver 301 and so on.
When the above-described method repeatedly transmits two OFDM symbols, computes a phase difference in the frequency domain by comparing phases of the symbols, and estimates a frequency offset from the computed phase difference, system throughput is degraded because the same data is repeatedly transmitted.
FIG. 4 is a schematic block diagram illustrating yet another conventional frequency offset estimator in the OFDM system that generates two received signals by oversampling one OFDM symbol at a double system clock, computes a phase difference by comparing phases of the two received signals, and estimates a frequency offset.
Referring to FIG. 4, an RF receiver 401 converts an RF signal received through an antenna (ANT) to a baseband frequency signal according to a frequency down-conversion process. An oversampler 402 oversamples the received signal output from the RF receiver 401 and generates two received signals constructed by an on-time sample and a delayed sample. A CP remover 403 removes CPs from the two received signals based on oversampling, extracts only transmitted data, and outputs the extracted transmitted data to a first phase compensator 404. The first phase compensator 404 compensates phases of the two received signals based on oversampling using a candidate frequency offset value selected by a controller 408. A Fourier transformer 405 transforms two time domain signals whose phases have been compensated to frequency domain signals according to a Fourier transform process, and outputs the frequency domain signals to a second phase compensator 406. The second phase compensator 406 compensates phases of the frequency domain signals using the candidate frequency offset value selected by the controller 408. A comparator 407 computes a difference between the two frequency domain signals whose phases have been compensated by the second phase compensator 406 using the candidate offset value. The controller 408 outputs a candidate frequency offset value in a search range to the first and second phase compensators 402 and 406, such that a repeat operation is performed. When a search is completed in the search range, the controller 408 outputs a frequency offset mapped to a minimum difference value as an actual frequency offset value. After the frequency offset output from the controller 408 is synchronized with a system clock, it is used to control the RF receiver 401 and so on.
When the above-described method oversamples a received signal, that is, one OFDM symbol, at a double system clock and computes a phase difference, a repeated pattern or pilot symbol is not required and therefore throughput is not degraded. However, because the above-described method requires an oversampling structure, hardware complexity of a receiving side increases.
Therefore, the conventional methods for estimating a frequency offset in an OFDM system are disadvantageous in that a system bandwidth is consumed due to pilot transmission or repeated data pattern transmission or hardware complexity of a receiving side increases.
A need exists for an efficient frequency offset estimation apparatus and method having performance equal to or better than that of the conventional method without consuming a system bandwidth and increasing hardware complexity to estimate a frequency offset in an OFDM system.